An issue in electrical devices, especially in lighting devices, and in particular in fluorescent and LED based light tubes (TLED) is to get the device certified and approved for commercial use. Such certification is, e.g. issued by the independent product safety certification organization Underwriters Laboratories (UL). A lighting device that potentially provides a risk of human exposure to high voltage, e.g. mains supply voltage, has to be harmless even when the lighting envelope is broken, either in full or in part.
For vacuum-sealed glass-based fluorescent tubes such demands are easily met. When the glass envelope breaks, the high voltage (HV) discharge between the tube's electrodes, located at the outer ends, can no longer be sustained. Firstly, since the vacuum has disappeared and secondly because of the loss of mechanical support/separation/clamping of the electrodes to the power terminals.
For other electrical devices, e.g. TLED tubes, however, carrying end-to-end spanning electronics and mechanical support, safety approval, e.g. according to UL-standards, comes at substantial costs. Moreover, since many tube designs comprise plastic envelopes, there is a higher risk of potential high voltage exposure simply because shell fracturing does not inherently imply mechanical and/or electric failure as well. Thus, more expensive solutions must be deployed to meet electrical safety demands, e.g. UL demands. There is thus a need to provide a cheap solution to provide an electrically safe electrical device that is safe also when its housing is broken.
To cope with electrical demands in, e.g., TLED tubes several options exist in the field such as, for example, the deployment of larger, more costly and less efficient low-voltage LED drivers, in combination with low-voltage LED strings. Furthermore, since the driver's input terminals are connected to mains voltage, the driver must be isolated with a wrap and be housed in a casing. In some cases, the casing is an aluminium half-tube, providing sufficient driver protection as well as mechanical support, heat spreading and heat-sinking. But, it also adds substantial weight and costs when compared to a light weight, vacuum “filled” and inexpensive glass tube as used for fluorescent light tubes. The other tube half, which serves as a light exit window, consists of a plastic.
Another option is to deploy a smaller, cheaper and more efficient isolated high-voltage LED driver. The driver may additionally be isolated by means of a plastic housing of 1.6 mm thickness that is inserted into a tube end of a TLED. In this case, the high voltage LED string is carried by a somewhat more expensive PCB having a high voltage coating and a larger track-to-ambient spacing (1.6 mm) leading to a wider PCB, thus adding costs. The PCB is laminated with a high voltage insulating adhesive film onto a metal rail. The high voltage insulating adhesive film is expensive. The thin rail is made of extruded aluminium and provides a recessed cavity towards the PCBs thereby providing additional high voltage protection by means of access restriction. Besides that, the rail acts as a heat-spreader, heat-sink and mechanical bridge between the tube ends. In this case the tube's exterior comprises a full plastic (diffuse) tube.
As may be deduced from the above, the main functions of the expensive metal bridge when compared to “free” vacuum are to provide tube rigidity, resistance to bending/sagging and torque, mechanical support for PCBs and return tracks, heat-spreading and heat-sinking and in some cases also an optical cavity. Despite all of the above, the fact is that anything added in between the end-caps also adds weight and costs. Thus, the cheapest tube would be one without LEDs, the fluorescent tubes being the only choice available in the state of the art technology. On the other hand, a green tube should not hold mercury, should be more energy efficient and be more sustainable, i.e. be made of glass but comprising efficient light sources such as LEDs. All this has to be achieved at a low cost.